(a) Field of the Invention
The present invention relates to a DC/DC voltage converter with multiple output voltages (hereinafter, it is called as multiple output DC/DC voltage converter) mainly applied to an active matrix-type liquid crystal display (LCD). More particularly, this invention relates to the multiple output DC/DC voltage converter combining a choke method with a fly-back method using a transformer and the LCD using thereof.
(b) Description of the Related Art
The DC/DC voltage converter converting DC supply voltage to a predetermined DC voltage having a different magnitude from the DC supply voltage is broadly used to a portable electronic equipment. Accordingly, this DC/DC voltage converter requires high efficiency, compactness and low price. Furthermore, since the portable electronic equipment commonly requires a plurality of DC supply voltages, the DC/DC voltage converter requires a plurality of output DC voltages.
FIG. 1 shows a pixel structure in the LCD.
As shown in FIG. 1, the pixel of the LCD comprises a pixel electrode formed in an area where a gate line crosses a data line, a common electrode facing the pixel electrode and a switch which is connected to the gate line, the data line and the pixel electrode, and is turned on/off responsive to a gate voltage applied through the gate line. The switch is made up of a thin film transistor (TFT). According to on/off state of the TFT, the switch applies or cut off a data supply voltage to the pixel electrode.
FIG. 2 shows waveforms of voltages applied to the pixel in FIG. 1.
In FIG. 2, the gate voltages have a gate on voltage Von for turning on the TFT and a gate off voltage Voff for turning off the TFT, and are applied to the gate line in FIG. 1. A common electrode voltage has a reverse phase with a data voltage. The common electrode voltage is applied to the common electrode and the data voltage is applied to the data line.
The common electrode voltage and the data voltage consume a significant amount of energy and require accurate voltages, and therefore, the common electrode voltage and the data voltage are provided from a main power supply. The gate on voltage Von and the gate off voltage Voff needs a small amount of energy, and therefore the gate on voltage Von and the gate off voltage Voff are provided from auxiliary power supplies.
In order to generate the voltages needed in driving the LCD, voltage generators using the choke method or the fly-back method are generally used.
FIG. 3 shows a voltage generator of the choke method.
As shown in FIG. 3, in the voltage generator of the choke method, a transistor Q1 periodically turns on or off responsive to a switching signal SW from switch control circuit 31. Consequently, it happens the current of an inductor L1 to change. The changing current causes a DC input voltage Vin to change into an AC(alternate current) voltage. The changed AC voltage is rectified by a diode D1 and a capacitor C1. At this time, the magnitude of a voltage charged in the capacitor C1, that is, an output voltage Vout depends on an on/off duty ratio of transistor Q1. The output voltage has a good efficiency. However, the voltage generator of the choke method produces only one output voltage. Therefore, there is a problem that the voltage generators are needed as many as DC voltages needed in an LCD for the purpose of applying to the LCD.
FIG. 4 shows a voltage generator of the fly-back method using the transformer.
As shown in FIG. 4, the voltage generator of the fly-back method has a transistor Q2 connected to a primary coil T1 of the transformer and pairs of diode-capacitors D2, C2: D3, C3: D4, C4 each connected to a plurality of secondary coils of the transformer T2, T3, T4.
In the voltage generator described in FIG. 4, the transistor Q2 periodically turns on or off responsive to a switching signal SW from a switch control circuit 41 and consequently it causes the current flowing through the primary coil T1 to change. The current change causes magnetic field to change and the change of magnetic field induces a voltage to the secondary coils T2, T3, T4 respectively. Accordingly, voltages induced in the secondary coils T2, T3, T4 are rectified to constant voltages by pairs of diode-capacitors and magnitude of the voltages charged in each capacitor C2, C3, C4 are determined by winding numbers of the primary and secondary coils. The voltages charged in the each capacitor C2, C3, C4 are used as the gate on voltage Von, the data supply voltage VDD and the gate off voltage Voff which are provided to the LCD module.
However, since the voltage generator of the fly-back method has a feature that the magnetic energy from the primary coil transfers to the secondary coil via magnetic core, there is a problem that it basically has a low efficiency and a big size of the transformer. Specifically, in case that the main power supply is generated using the voltage generator of the fly-back method, the size of the transformer becomes bigger.